Stepper motors are known to provide discrete angular motions of uniform magnitude, generally referred to as steps, rather than continuous rotation. Stepper motors are commonly utilized in printers, computer disk drives and other applications where it is necessary to move an element, such as printer head or magnetic transducer, to a predetermined position. When a predetermined current in the form of a motor drive pulse is applied to the stepper motor driver, it is assumed that the motor will rotate by one step to its next position, also referred to as null position, or detent. Enough power must be applied for each step to overcome the opposing torques present in the motor and the motor load. The motor is driven in such a way that after each step the polarity of the flux through the phases changes, thereby rotating the rotor to the next null position where it remains until the next flux change. The sequence in which the polarity for driving each phase changes may differ for each particular stepper motor type, and is prescribed by a phase driving timing diagram associated with each motor. Generally there may be about 100 steps per revolution, or less, depending on the number of magnetic poles. The rate of steps per unit of time is determined by the desired rate of rotation of the motor, or motor speed. The driving current necessary for each step is determined by the required load torque. Thus increasing the step current does not increase the rotational rate of the motor, nor does increasing the step rate increase the motor torque. Position sensors generally are not utilized with stepper motors to indicate the actual position of the rotor during operation. Such position sensors would normally indicate whether the rotor has actually moved after a drive pulse has been applied. Therefore, in stepper motors it is a general practice to apply a drive current of a somewhat greater magnitude than necessary to assure that the motor will make the required step in response to each motor drive pulse.
However, in applications where low power consumption is an important factor, it is desirable to apply only a required amount of drive current. It is also desirable to change the drive current with varying load conditions, while the step rate and step current must be controlled independently, so that a minimum amount of drive current may be maintained with changing operating conditions.